How to select vertices on the left side of a plane using python?
You need to somehow define what is 'on the left side'. Is it global space, is it local, which way you are looking from?.. Anyway, you can compare the coordinates to something. Let's say you are looking from the top as in the screenshot and left is the most negative X point of the object in global space plus 0.5 units:

import bpy

obj = bpy.context.object
data = obj.data

the_matrix = obj.matrix_world

on_the_left_side = (the_matrix @ obj.data.vertices[0].co).x # have to start somewhere

for v in data.vertices:
    co = the_matrix @ v.co #Let's say "on the left" is in global space
    if co.x &lt; on_the_left_side:
        on_the_left_side = co.x

offset = 0.5

#Element selection is a bit weird in bpy - you can select faces, 
#but not vertices for example like user cannot, so let's deselect
for e in data.edges:
    e.select = False
for f in data.polygons:
    f.select = False    

for v in data.vertices:
    co = the_matrix @ v.co
    if co.x &lt; on_the_left_side + offset:
        v.select = True
    else:
        v.select = False